NebGuide Nebraska Extension Research-Based Information That You Can Use G2295 · Index: Crops, Crop Production/Field Crops November 2017 Nutrient Management in Organic Farming Sam E. Wortman, Assistant Professor– Environmental Horticulture Charles S. Wortmann, Extension Soils Specialist Ashley Linville Pine, Former M.S. Graduate Student Charles A. Shapiro, Soils Specialist– Crop Nutrition Ashley A. Thompson, Post- Doctoral Research Associate Richard S. Little, Research Technologist Compost (left) and cover crops (right) are potentially complementary components of a successful organic nutrient management plan. This NebGuide addresses nutrient management in organic farming age plant and animal material applications to maintain or and considers sources, soil availability, and cycling of nutrients. improve soil organic matter,” but minimize “contamination of crops, soil, or water by nutrients, pathogenic organisms, Maintaining or enhancing long- term soil productiv- heavy metals, or residues of prohibited substances.” Any ity is a key provision of the National Organic Program soil- applied product, including composts or manures, must (NOP) regulation § 205.203, which states that the producer be produced in compliance with NOP regulations. must (a) select and implement practices “that maintain or The National Organic Standards Board (NOSB) improve the physical, chemical, and biological condition of periodically revises the NOP list of allowed or prohibited soil and minimize soil erosion,” (b) “manage crop nutrients substances by considering the need for the product and the and soil fertility through rotations, cover crops, and the impact it may have on human health and the environment. application of plant and animal materials,” and (c) “man- The Organic Materials Review Institute (OMRI) reviews 1 manufacturer claims for specific products annually to Table 1. Critical soil test values for four diverse field crops determine whether they meet NOP requirements. OMRI common to Nebraska (based on soil samples taken to an 8- maintains an extensive, up- to- date list of approved sub- inch depth). Values are derived from EC155, Nutrient Man- agement for Agronomic Crops in Nebraska stances available at www .omri .org. When a specific fer- Crop tilizer or amendment is not OMRI- listed, farmers should consult their third- party certifying agency prior to appli- Soil property Corn Dry beans Potato P (ppm) >16– 24 1 >10 2 >25 1 cation to ensure compliance. The use of organic materials, K (ppm) >125– 150 >125 >125 biofertilizers, cover crops, and crop rotations according to SO - S (ppm) >6– 8 3 * >8 National Organic Program standards are addressed in this 4 Zn (ppm) >0.8 >1.5 >1.0 NebGuide. 1. Bray- 1 P values; 2. Olsen- P values; dry bean production regions in Nebraska have Nutrient management on organic farms should high pH, calcareous soils where Olsen- P test is more appropriate (however, values can be economically meet crop nutrient needs and avoid soil multiplied by 1.5 to estimate Bray- 1 values); 3. Only applicable for sandy soil with very low soil organic matter; * = not limiting in soils of typical production regions nutrient depletion, while maintaining or improving soil productivity without excessive nutrient losses. Soil nutrient availability is dependent on diverse soil chemical, physi- cal, and biological properties, their interactions, and their organic cropping systems in Nebraska. Iron deficiency is interaction with the cropping system. While measurements commonly associated with calcareous soils with pH > 7.2. can be made for many soil properties, crop performance Alkaline soils on organic farms can be acidified over time is the best indicator of soil productivity. Farmers typically by applying OMRI- approved mineral sulfur (e.g., sulfate of manage to minimize soil physical and chemical constraints potash) or acidic mulches (e.g., pine needles). to sustainable productivity through practices such as: Applying agricultural lime to raise soil pH is allowed in organic production. Beef feedlot manure has a lim- • Applying organic materials such as manure, compost, ing effect, typically the equivalent of about 60 pounds of and biofertilizers to supply nutrients and maintain soil agricultural lime per ton of manure dry weight. However, organic matter most other manures and composts, and biological nitro- • Growing cover crops to cycle soil nutrients and biolog- gen fixation, have a net acidifying effect. Plant and soil ically fix atmospheric nitrogen testing laboratories commonly do recommend nutrient application rates, but Nebraska Extension EC155, Nutrient • Diversifying crop rotations for more efficient recovery Management for Agronomic Crops in Nebraska is especially and physiological use of valuable for interpretation of soil test results and determi- nation of nutrient and lime application rates. Critical levels for soil nutrient availability represent Soil and plant tissue analysis thresholds above which no yield increase is expected due to Most crop essential soil nutrients are adequately sup- application of the nutrient, although application of organic plied by agricultural soils in Nebraska. Application of nitro- materials may have other positive effects on the crop. See gen and phosphorus is most common for high crop produc- Table 1 for a sample of critical soil test values for several tivity in Nebraska. Deficiencies of potassium, sulfur (with essential plant nutrients for common field crops in Nebras- sandy soil), iron (with calcareous soil), and zinc can occur ka. These values can be used as a starting point for making and require attention in some fields. Yield increases due to annual decisions about nutrient sources and application application of other nutrients are rare in Nebraska. Manage- rates; however, developing a long- term nutrient manage- ment to avoid acidic soil conditions (Nebraska Extension ment plan and proactive (instead of reactive) management NebGuides G1503 and G1504) and soil aggregation, and to of soil fertility is advised for organic farms. avoid compaction (NebGuide G896) are also important for Soil nitrate- nitrogen is highly variable throughout ensuring sufficient nutrient availability to crops. a field and over time is not always a reliable predictor of Soil sampling every four years is advised to test for fertilizer need. Many farmers use a late- spring soil nitrate soil organic matter, pH, and availabilities of phosphorus, test to determine sidedress application rates in corn, but potassium, and zinc, as well as other properties of interest. sidedress fertilizer applications are rare in organic farm- Soil sampling practices and methods, such as grid versus ing. Crop nitrogen demand and annual removal should management zone sampling, are addressed in NebGuide guide fertilizer or amendment application rates in organic G1740. Soil pH 6 is the desired minimum pH for most farming, but rates can be reduced if nitrate is detected in 2 Table 2. A phosphorus (P) budget example for a four- year organic crop rotation where animal manure was applied to meet crop nitrogen demand, resulting in high phosphorus application and accumulation over time P content Rate Total P content Crop Total P Total P Crop Inputs of inputs (dry) P input of crop* yield removed† balance % tons/ac lb/ac % lb/ac lb/ac lb/ac Soybean None 0.659 1,775 12 - 12 Corn Manure 0.6 15 180 0.317 5,850 19 161 Soybean None 0.659 1,775 12 - 12 Wheat Manure 0.6 10 120 0.435 2,810 12 108 4- yr total 245 *Based on values from NRCS Plant Nutrient Content Database. All P values are for elemental P and not for P2O5; † Total P removed = (% P content of grain) x (grain yield) soil samples taken to the rooting depth (2– 4 feet for most crops but contributes to phosphorus loss in runoff and ero- crops; see EC155 for interpretation of deep soil nitrate val- sion and contamination of water bodies. Therefore, manure ues in agronomic crops) and when a legume is the preced- nitrogen needs to be complemented by biological nitrogen ing crop (see section below for further information). fixation or other nitrogen sources in organic systems. In addition to increased productivity, using organic A nutrient budget can be a useful tool for long- term materials to fertilize crops can improve broader measures accounting of phosphorus addition via manure (or other of soil “quality” or “health.” Changes to soil physical and fertilizer source) and removal via harvested crops (see ex- biological properties that may result from long- term or- ample in Table 2). This approach to nutrient management ganic nutrient management include reduced bulk density, can help to ensure organic farmers do not unsustainably increased aggregate size and stability, increased water in- mine inherent soil nutrient reserves, and on the other end filtration, and increased water- holding capacity; increased of the extreme, do not build up soil nutrient levels of high microbial biomass, diversity, and respiration; and increased risk for environmental contamination. nitrogen mineralization potential, reduced soil erosion The National Organic Program restricts the use of ma- and soil crusting, and increased soil carbon sequestra- nure on crops for human consumption. Raw manure must tion (NebGuide G2283). Soil physical and biological tests be applied more than 120 days before harvest if the edible are available at some soil testing labs. Other “soil health” portion of the crop will be in direct contact with soil, and do- it- yourself procedures also are available, including the 90 days before